During the early 80's, Maiden was part of the "NWOBHM"; New Wave of British Heavy Metal - but this was more of a scene than a style of music. I don't think I've heard Maiden described as anything other than "heavy metal" in a strictly musical sense. They and Judas Priest pretty much defined what heavy metal was for a long time.
Genres are handy sometimes, but there are some bands that won't fit into any genre. Such as Slayer, Therion and Nightwish. Most of the time I think slapping a label onto such music is totally redundant. Such bands must be heard, not explained.
'bane